This invention relates generally to natural gas transfer systems and, more particularly, to methods and apparatus for transporting natural gas through a pipeline.
Gas compression is needed in the chemical, oil and gas industry, mainly for pumping natural gas from on-shore or off-shore wells to processing plants, for subsequent gas transmission or for collection in storage facilities for use in peak hours. In at least some other applications, gas compression is also needed for downstream applications in hydrocarbon processing industries and chemical industries, and to facilitate distribution to gas end-users.
Natural gas typically includes methane as the principle constituent and may also include other substances including impurities. Natural gas pipeline compressors are conventionally driven by gas turbines, sub-synchronous motors with a gearbox, and/or by high-speed directly coupled induction or synchronous motors. Known sub-synchronous motors have an in-service rotor speed of less than 3,000 revolutions per minute (rpm) for 50 Hz electrical power supplies and less than 3,600 rpm for 60 Hz electrical power supplies. Known synchronous motors have an in-service rotor speed of approximately 3,000 rpm for 50 Hz electrical power supplies and approximately 3,600 rpm for 60 Hz electrical power supplies. Known super-synchronous motors have an in-service rotor speed of greater than 3,000 rpm for 50 Hz electrical power supplies and greater than 3,600 rpm for 60 Hz electrical power supplies.
Electric drives (motors) may be advantageous over mechanical drives (gas turbines) in operational flexibility (variable speed), maintainability, reliability, lower capital cost and lower operational cost, better efficiency and environmental compatibility. Additionally, electric drives generally require a smaller foot print, are easier to integrate with the compressor, and have the potential for higher reliability than mechanical drives. For example, some known electric drives do not utilize a gearbox to facilitate an increased compressor speed and as such are generally simpler in construction than mechanical drives. Super-synchronous electric drives may increase operating efficiency by operating with an increased speed that facilitates compressing the transported gas more rapidly.
However, electric drives may be more difficult to seal. For example, some known super-synchronous electric drives do not utilize external gas seals. In addition, at least some known super-synchronous electric drives utilize an internal seal system, i.e. dry gas seals, to facilitate sealing the process gas from the environment. However, the complexity of such sealing systems may lead to reduced availability and increased maintenance costs. Moreover, such seal systems may be prone to produce leakage either to the process gas (contamination) or to the environment (flaring).